The invention relates to a method for the synthesis of complexes of platinum and alkenylpolysiloxane, and especially to platinum divinyltetramethyldisiloxane, which is synthesized by this method, and to several uses.
In the EP 0 979 837 A2, a method is disclosed for the synthesis of a platinum hydrosilylation catalyst, for which initially a mixture of a haloplatinic acid, an alkenylpolysiloxane and a cycloalkylpolysiloxane is stirred at a particular temperature, after which the essentially the halogen is removed with sufficient alkali and the mixture is filtered, in order to obtain the corresponding platinum hydrosilylation catalyst. Moreover, 0.3 to about 20 moles of cycloalkylpolysiloxane per mole of alkenylpolysiloxane must be made available.
It is a disadvantage of this method that, for example, the platinum divinyltetramethyldisiloxane formed has a dark color, which does not meet the requirements (colorless to slightly yellow), which are required for certain specifications.
From the above, the problem arises of employing a novel method for synthesizing platinum alkenylpolysiloxane complexes or platinum divinyltetramethyldisiloxane, which eliminates the disadvantages named above at least partly, to make available especially a relatively inexpensive method for synthesizing the above compound, for which the platinum alkenylpolysiloxane complexes, especially the platinum divinyltetramethyldisiloxane, are almost colorless compounds.
This problem is solved by the method of the present invention. For the inventive method, initially a haloplatinum compound is reacted with at least one alkenylpolysiloxane with the addition of platinum in the form of a platinum complex as autocatalyst and with the addition of at least one base, especially of a hydrogen carbonate, carbonate or hydroxide, at the lowest possible temperature, especially at a temperature below +60xc2x0 C., in an organic solvent mixture or solvent mixture, which can be oxidized by the haloplatinum compound.
As a rule, the reaction formulation is stirred during the reaction, for example, by means of a magnetic stirrer, for better mixing and in order to avoid local overheating. The individual reactants are added to the reaction formulation, for example, in portions or dropwise.
It was a surprising finding of essential importance to the invention that, upon adding small amounts of platinum alkenylpolysiloxane, the reaction, as such, is autocatalyzed so that the usual reaction temperature can be lowered by up to about 20xc2x0 K. As a result, there is less decomposition and fewer byproducts are formed, so that the actual product is also contaminated less in color.
For a first variation of the inventive method, activated charcoal is added before the reaction. After the reaction, the activated charcoal suspension obtained is concentrated and, subsequently, the solid particles and activated charcoal are separated from a first liquid phase, preferably by filtration, or the solid particles and activated charcoal are separated from a first liquid phase and, subsequently, the first liquid phase is concentrated. The reaction product, a platinum alkenylpolysiloxane, is contained in the liquid phase.
In a second variation of the inventive method, the solid particles are separated from a first liquid phase after the reaction. Subsequently, the first liquid phase is treated with activated charcoal to form an activated charcoal suspension. After that, the suspension is concentrated and the activated charcoal is separated from the first liquid phase or the activated charcoal is separated from the first liquid phase and, subsequently, the first liquid phase is concentrated.
For a third variation of the inventive method, the reaction suspension obtained after the reaction is treated with activated charcoal to form an activated charcoal suspension. Subsequently, the activated charcoal suspension is concentrated, whereupon the solid particles and the activated charcoal are separated from a first liquid phase or the solid particles and activated charcoal are separated from a first liquid phase and, subsequently, the first liquid phase is concentrated.
It is of advantage if, for the reaction steps of the three alternative embodiments based on the platinum used as a haloplatinum compound, at least 1 percent by weight of platinum is used in the form of a platinum complex as autocatalyst.
Advantageously, the alkenylpolysiloxane is added first to the solvent or solvent mixture. This is followed by the addition of the haloplatinum compound. If the addition is reversed, unstable solutions may be formed, which may still react even days after they are formed.
For the inventive method, divinyltetramethyldisiloxane has proven its value in practice as alkenylpolysiloxane and hexachloroplatinic acid has proven its value in practice as haloplatinum compound.
Moreover, it has proven to be advantageous to wash the activated charcoal with the solvent or solvent mixture and to combine the second liquid phase obtained with the first liquid phase, in order to increase the yield of product.
Advantageously, it has proven to be of value in practice to use at least one alcohol, especially isopropanol and/or ethanol as solvent or solvent mixture.
If the haloplatinum compound is added as an anhydrous solution to the reaction formulation, especially as a solution of the haloplatinum compound in isopropanol, a controlled evolution of carbon dioxide gas takes place advantageously which, in turn, limits the discharge of the solvent by gas evolution, so that it is also possible to adhere to the strict emission limits.
Finally, it has proven to be advantageous in practice to carry out the reaction at a temperature of +42xc2x0 C. to +58xc2x0 C. and especially at a temperature of +48xc2x0 C. to +52xc2x0 C.
The alkenylpolysiloxane, especially the platinum divinyltetramethyldisiloxane, synthesized with this method, is almost colorless in comparison with the substances synthesized by methods known from the state of the art and has an iodine color number of 0 to 15 (measured according to DIN 6162).
Also of importance for the positive properties is the autocatalytic use of the actual product for synthesizing the same, since the reaction temperature can be lowered by up to about 20xc2x0 K in this way, as a result of which fewer decomposition products and byproducts are formed and the reaction product is less contaminated.
The following examples explain the invention.